Assessing the Quality of Municipal Yard Trimmings

Dan M. Sullivan

Source-separated municipal yard trimmings (grass, leaves, tree and shrub prunings) are widely available in urban fringe areas west of the Cascade Mountains in the Pacific Northwest. Major nutrients provided by yard trimmings are nitrogen (N) and potassium (K). Yard trimmings also provide organic matter to improve soil tilth. Composted yard trimmings can be used as a substitute for other organics (e.g. bark and peat moss) in potting media.

Compost facility managers are looking for alternatives to deal with a glut of grassy yard trimmings that arrives at compost facilities west of the Cascades in spring and early summer. This problem in the urban sector provides an opportunity for economical field application of "minimally processed" yard trimmings. Minimally processed yard trimmings are ground to reduce particle size and allowed to heat in piles for a week or so to kill weed seeds. They are offered to growers at low cost.

Finished composts are typically produced by municipal composting facilities after 40 to 120 days of managed composting. Finished yard trimmings composts are expensive to produce. Most of the finished yard trimmings compost used in agriculture is utilized for production of high-value crops (e.g. potting media). Finished compost for high value crops must be consistent in maturity and stability. Simple measures of compost maturity and stability are highly desired by composters and compost users.

Our research project works directly with composters and compost users to develop niche markets for yard trimmings. The primary objectives of our current research are to: 1) quantify plant-available N provided by minimally-processed yard trimmings, and 2) develop appropriate quick test procedures to assess the maturity and stability of composted yard trimmings.

Methods:

1. N release from minimally-processed yard trimmings

We measured short-term N mineralization/immobilization dynamics following yard trimmings incorporation into soil in the laboratory under controlled temperature and moisture conditions. We are using this laboratory incubation data to develop a simple model based on growing degree-days. Our model predicts the percentage of yard trimmings total N that is mineralized to plant-available forms (ammonium and nitrate-N) during the first growing season after application. Key indicators of yard trimmings quality used in the model are C:N ratio, moisture, and organic matter content.

2. Maturity and stability of finished compost

Active compost piles were sampled repeatedly over a 90-day period. We assessed yard trimmings compost maturity and stability via biological tests (respiration, self-heating, seed germination and growth) and chemical tests (pH, electrical conductivity, nitrate-N, ammonium-N, C:N ratio, and cation exchange capacity).

Results and Discussion:

1. N release from minimally processed yard trimmings

The quantity of plant-available N released following soil incorporation of minimally processed yard trimmings varied with C:N ratio. For all minimally processed yard trimmings, an initial period of immobilization of plant available N (14 to 21 days at 25°C) was followed by a period of net mineralization at a slow rate. Typical yard trimmings with C:N of 15 to 20:1 (1.4 to 1.8 % total N on a dry weight basis) had net N mineralization near zero (-5 to + 5 % percent of total N applied) in a 77 day incubation at 25 °C. These findings are being incorporated into our computer model. Further laboratory and field studies are needed to calibrate and verify the accuracy of the model.

2. Maturity and stability of finished compost

Respiration rate (carbon dioxide evolution rate) was the most reliable test for compost maturity and stability. Results from quick tests were strongly correlated with established laboratory procedures for measuring respiration rate. The pH of yard trimmings compost was also highly correlated to compost stability. During the early stages of decomposition, pH values were 5 to 6, rising to near pH 7 by the end of the composting process. This change in pH was probably related to the formation of organic acids in the early stages of composting, followed by degradation of organic acids as the compost matured.

Conclusions:

Yard trimmings are a valuable resource for sustainable agricultural systems. We anticipate further improvements in test procedures to evaluate product quality. Our initial work with minimally processed yard trimmings indicates that they do not have a large impact on short-term N availability. Simple compost respiration rate test procedures are now available. They provide a quick assessment of the maturity and stability of finished compost.

This research is a joint project of Oregon State and Washington State Universities. WSU scientists Andy Bary and Craig Cogger are co-leaders of the project. Theresa Cramer and Linda Brewer, graduate students at Oregon State University, are conducting project research in partial fulfillment of M.S. degree requirements. Snohomish and King Counties (WA), and Land Recovery Inc. (Puyallup, WA) provided financial support.

Dan M. Sullivan
Soil Scientist
Oregon State University
3017 ALS
Corvallis, OR 97331
Tel: (541) 737-5715
Dan.Sullivan@orst.edu

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